This invention relates to a method for slip compensation in a squirrel cage induction motor when the motor is fed by an inverter without feedback at a supply frequency less than 20% of the rated supply frequency of the motor after the motor has first been fed at a frequency more than 50% of the rated frequency for a period of time at least equal to a mechanical time constant of a combination formed by the motor and a load device, wherein a term proportional to the torque of the motor is determined and added to the set value of the rotation speed to effect slip compensation.
Techniques of the type described above are used for the compensation of the slip of squirrel cage induction motors particularly in lifts, conveyors, cranes and trains, in which the motor is normally driven at a greater frequency typically more than 50% of the rated frequency of the motor, but which are driven at a low drift speed before the motor is switched off, whereby the supply frequency of the motor is typically less than 20% of the rated frequency of the motor. In this way, stopping can be performed smoothly. If the system does not comprise position feedback or speed feedback e.g. from a tachometer measuring the rotation speed of the motor, speed variation caused by the slip of the motor is to be compensated for electrically. Such a system can be e.g. a positioning drive without feedback information from the motor shaft, where a short distance is driven at a low approach speed which is to be load-independent before positioning in order to improve accuracy.
Power Electronics, Mohan, Undelend, Robbins, John Wiley & Sons, New York 1989, page 335, describes a method for electric compensation of the slip of a squirrel cage induction motor, where one aims to keep the rotation speed of the motor independent of the load by adding a term proportional to the torque (T) of the motor to the set value of the rotation speed. The stator frequency (fs) can thus be determined from the equation EQU fs=(p/2) (n+kT)/60 (1),
where
p=the pole number of the motor PA1 n=the set value of the rotation speed (r/min) PA1 k=a constant dependent on the degree of magnetization of the machine PA1 fs=the stator frequency (Hz) PA1 Pdc=a power measured from the intermediate circuit PA1 Pinv=the dissipation power of the inverter PA1 Ps=the dissipation power of the stator of the motor. PA1 Ws=the angular frequency of the stator.
The term kT in Equation (1) is obtained by an estimated air-gap power Pa obtained by a power measured from an intermediate circuit between a rectifier and an inverter as follows: EQU Pa=Pdc-Pinv-Ps (2),
where
The torque of the motor in turn is obtained from the air-gap power Pa as follows: EQU T=Pa/Ws (3),
where
However, the accuracy of the above-described method decreases with decreasing frequency, as the proportion of the dissipation powers Pinv and Ps in the measured power of the intermediate circuit increases with decreasing frequency when the load remains constant. When the frequency is 0, the entire power of the intermediate circuit is lost through losses as the air-gap power Pa of the machine is 0. Moreover, in practice, changes in the degree of magnetization caused by the voltage loss of the stator resistance alter the value of the constant k, thus deteriorating the accuracy of the term kT the more the lower the frequency is.